Mechanism of inhibition of ribonucleoside diphosphate reductase by a-(N)-heterocyclic aldehyde thiosemicarbazones

Desulfurization of thiosemicarbazones: the role of metal ions and biological implications

Thiosemicarbazones are biologically active substances whose structural formula is formed by an azomethine, an hydrazine, and a thioamide fragments, to generate a R2C=N-NR-C(=S)-NR2 backbone. These compounds often act as ligands to generate highly stable metal-organic complexes. In certain experimental conditions, however, thiosemicarbazones undergo reactions leading to the cleavage of the chain. Sometimes, the breakage involves desulfurization processes. The present work summarizes the different chemical factors that influence the desulfurization reactions of thiosemicarbazones, such as pH, the presence of oxidant reactants or the establishment of redox processes as those electrochemically induced, the effects of the solvent, the temperature, and the electromagnetic radiation. Many of these reactions require coordination of thiosemicarbazones to metal ions, even those present in the intracellular environment. The nature of the products generated in these reactions, their detection in vivo and in vitro, together with the relevance for the biological activity of these compounds, mainly as antineoplastic agents, is discussed.

Research Progress on the Biological Activities of Metal Complexes Bearing Polycyclic Aromatic Hydrazones

Due to the abundant and promising biological activities of aromatic hydrazones, it is of great significance to study the biological activities of their metal complexes for the research and development of metal-based drugs. In this review, we focus on the metal complexes of polycyclic aromatic hydrazones, which still do not receive much attention, and summarize the studies related to their biological activities. Although the large number of metal complexes in phenylhydrazone prevent them all from being summarized, the significant value of polycyclic aromatic hydrocarbons themselves (such as naphthalene and anthracene) as pharmacophores are also considered. Therefore, the bioactivities of the metal complexes of naphthylhydrazone and anthrahydrazone are focused on, and the recent research progress on the metal complexes of anthrahydrazone by the authors is also included. In terms of biological activities, these complexes mainly show antibacterial and anticancer activities, along with less bioactivities. The present review demonstrates that the structural design and bioactivities of these complexes are fundamental, which also indicates a certain structure-activity relationship (SAR) in some substructural areas. However, a systematic and comprehensive conclusion of the SAR is still not available, which suggests that more attention should be paid to the bioactivities of the metal complexes of polycyclic aromatic hydrazones since their potential in structural design and biological activity remains to be explored. We hope that this review will attract more researchers to devote their interest and energy into this promising area.

Thiosemicarbazones as Potent Anticancer Agents and their Modes of Action

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Thiosemicarbazones (TSCs) are a class of Schiff bases usually obtained by the condensation of thiosemicarbazide with a suitable aldehyde or ketone. TSCs have been the focus of chemists and biologists due to their wide range of pharmacological effects. One of the promising areas in which these excellent metal chelators are being developed is their use against cancer. TSCs have a wide clinical antitumor spectrum with efficacy in various tumor types such as leukemia, pancreatic cancer, breast cancer, non-small cell lung cancer, cervical cancer, prostate cancer and bladder cancer. To obtain better activity, different series of TSCs have been developed by modifying the heteroaromatic system in their molecules. These compounds possessed significant antineoplastic activity when the carbonyl attachment of the side chain was located at a position α to the ring nitrogen atom, whereas attachment of the side chain β or γ to the heterocyclic N atom resulted in inactive antitumor agents. In addition, replacement of the heterocyclic ring N with C also resulted in a biologically inactive compound suggesting that a conjugated N,N,S-tridentate donor set is essential for the biological activities of thiosemicarbazones. Several possible mechanisms have been implemented for the anticancer activity of thiosemicarbazones.

Synthesis, NMR structural characterization and molecular modeling of substituted thiosemicarbazones and semicarbazones using DFT calculations to prove the syn/anti isomer formation

Thiosemicarbazones possessing electron-donating and electron-withdrawing groups were prepared, and their spectral characteristics determined. In all cases, the spectra showed that one isomer was formed, allowing further functionalization to molecules of biological interest. We provide NMR data for some of the thiosemicarbazones and semicarbazones. We also provide evidence that for 2-pyridyl thiosemicarbazone, the syn isomer slowly converts into the anti isomer in dimethyl sulfoxide solvent with first-order kinetics. Molecular modeling and density functional theory calculations confirmed these observations.

Thiosemicarbazones: the new wave in cancer treatment

The anticancer effects of thiosemicarbazones were once solely attributed to the inhibition of ribonucleotide reductase, an enzyme involved in the rate-limiting step of DNA synthesis. However, the mechanism behind this inhibition was initially not described. The ability of thiosemicarbazones to chelate metal ions has now been recognized as a major factor in their antiproliferative effects. This mini-review discusses current advances of an emerging 'new wave' of thiosemicarbazones as potent anticancer agents, describing recent insights into their mechanism of action. The redox activity of Fe-thiosemicarbazone complexes is critical in their anticancer activity, resulting in oxidative damage and the inhibition of ribonucleotide reductase. In vivo analysis indicates that some thiosemicarbazones show potential as chemotherapeutic agents.

Thiosemicarbazone derivate protects from AAPH and Cu2+ -induced LDL oxidation

AIMS

Several lines of evidence support the hypotheses that the oxidation of low density lipoprotein (LDL) may play a crucial role in the initiation and progression of atherosclerosis. Oxidative stress is one of the causes of the overproduction of reactive species that increase the formation of oxidized LDL. Thiosemicarbazones are compounds used in anticancer, antiviral and antifungal therapy; however, its redox activity has been controversial. Thus, we tested, in vitro, a possible antioxidant activity of a thiosemicarbazone derivate, the isatin-3-N(4)-benzilthiosemicarbazone (IBTC).

MAIN METHODS

We measured the conjugated diene formation in serum and LDL as well as the loss of tryptophan fluorescence in LDL induced by two oxidant agents, 2,2-azobis(2-amidinopropane dihydrochloride) (AAPH) and Cu(2+). Thiobarbituric acid reactive substances (TBARS) formation in LDL and in different rat tissues was also assessed. The toxicity of IBTC was measured using aortic slices viability assay.

KEY FINDINGS

Our results show that IBTC significantly reduced the AAPH and Cu(2+)-induced formation of conjugated dienes, increased in a dose-dependent manner the lag phase and the t(1/2) of tryptophan fluorescence, and reduced the TBARS formation in LDL, plasma and rat tissues, showing no toxicity to aortic slices.

SIGNIFICANCE

These results indicate that IBTC is a good antioxidant and a promising antiatherogenic agent for further studies in vivo.

Cytotoxic Evaluation of 3-Aminopyridine-2-Carboxaldehyde Thiosemicarbazone, 3-AP, in Peripheral Blood Lymphocytes of Patients with Refractory Solid Tumors using Electron Paramagnetic Resonance

PURPOSE: 3-AP (3-aminopyridine-2-carboxaldehyde thiosemicarbazone, 3-AP) is a metal chelator that potently inhibits the enzyme ribonucleotide reductase, RR, which plays a key role in cell division and tumor progression. A sub-unit of RR has a non-heme iron and a tyrosine free radical, which are required for the enzymatic reduction of ribonucleotides to deoxyribonucleotides. The objective of the study was to determine whether 3-AP affects its targeted action by measuring EPR signals formed either directly or indirectly from low molecular weight ferric-3-AP chelates. METHODS: Peripheral blood lymphocytes were collected from patients with refractory solid tumors at baseline and at 2, 4.5 and 22 hours after 3-AP administration. EPR spectra were used to identify signals from high-spin Fe-transferrin, high-spin heme and low-spin iron or copper ions. RESULTS: An increase in Fe-transferrin signal was observed, suggesting blockage of Fe uptake. It is hypothesized that formation of reactive oxygen species by FeT(2) or CuT damage transferrin or the transferrin receptor. An increase in heme signal was also observed, which is a probable source of cytochrome c release from the mitochondria and potential apoptosis. In addition, increased levels of Fe and Cu were identified. CONCLUSION: These results, which were consistent with our earlier study validating 3-AP-mediated signals by EPR, provide valuable insights into the in vivo mechanism of action of 3-AP.

Synthesis, cytotoxic and antimalarial activities of benzoyl thiosemicarbazone analogs of isoquinoline and related compounds

Thiosemicarbazone analogs of papaveraldine and related compounds 1–6 were synthesized and evaluated for cytotoxic and antimalarial activities. The cytotoxic activity was tested against HuCCA-1, HepG2, A549 and MOLT-3 human cancer cell lines. Thiosemicarbazones 1–5 displayed cytotoxicity toward all the tested cell lines, while compounds 2–5 selectively showed potent activity against the MOLT-3 cell lines. Significantly, N(4)-phenyl-2-benzoylpyridine thiosemicarbazone 4 exhibited the most potent activity against HuCCA-1, HepG2, A549 and MOLT-3 cell lines with IC₅₀ values of 0.03, 4.75, 0.04 and 0.004 µg/mL, respectively. In addition, 2-benzoylpyridine thio-semicarbazones 3 and 4 showed antimalarial activity against Plasmodium falciparum with IC₅₀ of 10^-7 to < 10^-6 M. The study demonstrates the quite promising activity of analog 4 as a lead molecule for further development.

A hexadentate bis(thiosemicarbazonato) ligand: rhenium(V), iron(III) and cobalt(III) complexes

A new 1,3-diaminopropane bridged bis(thiosemicarbazone) ligand (H(4)L) has been synthesised. The new hexadentate ligand is capable of forming six coordinate complexes with rhenium(V), iron(III) and cobalt(III). In the case of the iron(III) and cobalt(III) complexes the ligand doubly deprotonates to give the monocations [Fe(III)(H(2)L)](+) and [Co(III)(H(2)L)](+) in which the metal ion is in a distorted octahedral environment. In the rhenium(V) complex the ligand loses four protons by deprotonation of both secondary amine nitrogen atoms to give [Re(V)(L)](+) with the metal ion in a distorted trigonal prismatic coordination environment. [Re(V)(L)](+) represents a rare example of a rhenium(V) complex that does not contain one of the ReO(3+), ReN(2+) or Re(NPh)(2+) cores. The new ligand and metal complexes have been characterised by a combination of NMR spectroscopy, X-ray crystallography, mass spectrometry and microanalysis. The electrochemistry of [Fe(III)(H(2)L)](+), [Co(III)(H(2)L)](+) and [Re(V)(L)](+) has been investigated by cyclic voltammetry with each complex undergoing a single electron reduction event. It is possible to prepare the rhenium(V) complex from ReOCl(3)(PPh(3))(2) or directly from [ReO(4)](-) with the addition of a reductant, which suggests the new ligand may be of interest in the development of rhenium radiopharmaceuticals.

Cancer cell iron metabolism and the development of potent iron chelators as anti-tumour agents

Cancer contributes to 50% of deaths worldwide and new anti-tumour therapeutics with novel mechanisms of actions are essential to develop. Metabolic inhibitors represent an important class of anti-tumour agents and for many years, agents targeting the nutrient folate were developed for the treatment of cancer. This is because of the critical need of this factor for DNA synthesis. Similarly to folate, Fe is an essential cellular nutrient that is critical for DNA synthesis. However, in contrast to folate, there has been limited effort applied to specifically design and develop Fe chelators for the treatment of cancer. Recently, investigations have led to the generation of novel di-2-pyridylketone thiosemicarbazone (DpT) and 2-benzoylpyridine thiosemicarbazone (BpT) group of ligands that demonstrate marked and selective anti-tumour activity in vitro and also in vivo against a wide spectrum of tumours. Indeed, administration of these compounds to mice did not induce whole body Fe-depletion or disturbances in haematological or biochemical indices due to the very low doses required. The mechanism of action of these ligands includes alterations in expression of molecules involved in cell cycle control and metastasis suppression, as well as the generation of redox-active Fe complexes. This review examines the alterations in Fe metabolism in tumour cells and the systematic development of novel aroylhydrazone and thiosemicarbazone Fe chelators for cancer treatment.

Electron paramagnetic resonance study of peripheral blood mononuclear cells from patients with refractory solid tumors treated with Triapine

The metal chelator Triapine, 3-aminopyridine-2-carboxaldehyde thiosemicarbazone, is a potent inhibitor of ribonucleotide reductase. EPR spectra consistent with signals from Fe-transferrin, heme, and low-spin iron or cupric ion were observed in peripheral blood mononuclear cells (PBMCs) obtained from patients treated with Triapine. One signal that is unequivocally identified is the signal for Fe-transferrin. It is hypothesized that Fe uptake is blocked by reactive oxygen species generated by FeT(2) or CuT that damage transferrin or transferrin receptor. A potential source for the increase in the heme signal is cytochrome c released from the mitochondria. These results provide valuable insight into the in vivo mechanism of action of Triapine.

Neuroprotective activity of 3-aminopyridine-2-carboxaldehyde thiosemicarbazone (PAN-811), a cancer therapeutic agent

3-aminopyridine-2-carboxaldehyde thiosemicarbazone (3-AP) is a highly-hydrophobic small molecule that was originally developed for cancer therapy (Triapine, Vion Pharmaceuticals) due to its ability to inhibit ribonucleotide reductase, a key enzyme required for DNA synthesis. 3-AP has a high affinity for divalent cations, chelating the Fe(2+) at the R2 subunit of the enzyme and inhibiting formation of a tyrosyl radical essential for ribonucleotide reduction. We have demonstrated that 3-AP is also a potent neuroprotectant (as such, it is referred to as "PAN-811"). In vitro it completely blocks ischemic neurotoxicity at a concentration of 0.5 microM (EC(50) approximate, equals 0.35 microM) and hypoxic toxicity at 1.2 microM (EC(50) approximate, equals 0.75 microM). Full protection of primary cortical and striatal neurons can be achieved with 3-AP when it is added to the medium at up to six hours after an ischemic insult. 3-AP also suppresses cell death induced by neurotoxic agents, including staurosporine, veratridine and glutamate, indicating activity against a central target(s) in the neurodegenerative process. 3-AP acts via neutralization of two important intracellular effectors of excitatory neurotoxicity; calcium and free radicals. Its reported ability to elevate anti-apoptotic proteins is likely to be a consequence of the suppression of excessive intracellular free calcium. In a rat model of transient ischemia, a single bolus delivery of 3-AP 1 h after the initiation of ischemic attack reduced infarct volume by 59% when administered i.c.v. (50 mug per rat) and by 35% when administered i.v. (1 mg/kg). In Phase I clinical trials in cancer therapy 3-AP had no cardiovascular, CNS or other major adverse effects. Thus, 3-AP has a high potential for development as a novel, potent neuroprotectant for the treatment of neurodegenerative diseases.

The Evolution of Iron Chelators for the Treatment of Iron Overload Disease and Cancer

The evolution of iron chelators from a range of primordial siderophores and aromatic heterocyclic ligands has lead to the formation of a new generation of potent and efficient iron chelators. For example, various siderophore analogs and synthetic ligands, including ICL670A [4-[3,5-bis-(hydroxyphenyl)-1,2,4-triazol-1-yl]-benzoic acid], 4'-hydroxydesazadesferrithiocin, and Triapine, have been developed from predecessors and illustrate potent iron-mobilizing or antineoplastic activities. This review focuses on the evolution of iron chelators from initial lead compounds through to the development of novel chelating agents, many of which show great potential to be clinically applied in the treatment of iron overload disease and cancer.

Therapeutic potential of iron chelators in cancer therapy

The success of DFO at markedly inhibiting the growth of aggressive tumors such as neuroblastoma and leukemia justifies interest in the development of chelators as anti-neoplastic agents. This is emphasized by the fact that DFO has suboptimal properties, namely poor membrane permeability and a very short serum half-life. More recently, the thiosemicarbazone chelator, Triapine, has entered a phase I clinical trial again confirming the potential of these compounds. Further studies examining the effects of chelators on neoplastic cells will not only be valuable in terms of identifing novel anti-cancer agents, but will also provide new information on the role of Fe in cell cycle control.

Iron chelators for the treatment of iron overload disease: relationship between structure, redox activity, and toxicity

The success of the iron (Fe) chelator desferrioxamine (DFO) in the treatment of beta-thalassemia is limited by its lack of bioavailability. The design and characterization of synthetic alternatives to DFO has attracted much scientific interest and has led to the discovery of orally active chelators that can remove pathological Fe deposits. However, chelators that access intracellular Fe pools can be toxic by either inhibiting Fe-containing enzymes or promoting Fe-mediated free radical damage. Interestingly, toxicity does not necessarily correlate with Fe-binding affinity or with chelation efficacy, suggesting that other factors may promote the cytopathic effects of chelators. In this review, we discuss the interactions of chelators and their Fe complexes with biomolecules that can lead to toxicity and tissue damage.

Iron chelators as therapeutic agents for the treatment of cancer

A wide variety of studies in vitro, in vivo, and in clinical trials have demonstrated that the chelator currently used to treat iron overload disease, desferrioxamine, has anti-proliferative effects against both leukemia and neuroblastoma. However, the efficacy of desferrioxamine is severely limited due to its poor ability to permeate cell membranes and chelate intracellular iron pools. These studies have led to the development of other iron chelators that are far more effective than desferrioxamine. Some of these chelators such as 3-aminopyridine-2-carboxaldehyde thiosemicarbazone (Triapine) have entered phase I clinical trials, while other chelators such as 2-hydroxy-1-naphthylaldehyde isonicotinoyl hydrazone or tachpyridine require evaluation in animal models. The high anti-tumor activity observed with these ligands certainly suggests further development of chelators as anti-cancer agents is warranted.

Inhibitors of ribonucleotide reductase. Comparative effects of amino- and hydroxy-substituted pyridine-2-carboxaldehyde thiosemicarbazones

A new series of alpha-(N)-heterocyclic carboxaldehyde thiosemicarbazones (HCTs) was studied for their effects on L1210 cell growth in culture, cell cycle transit, nucleic acid biosynthesis and ribonucleotide reductase activity. 3-Aminopyridine-2-carboxaldehyde thiosemicarbazone (3-AP) and 3-amino-4-methylpyridine-2-carboxaldehyde thiosemicarbazone (3-AMP) were the most active compounds tested with respect to inhibition of cell growth and ribonucleotide reductase activity. 5-Aminopyridine-2-carboxaldehyde thiosemicarbazone (5-AP) and 4-methyl-5-aminopyridine-2-carboxaldehyde thiosemicarbazone (5-AMP) were slightly less active. 3-AP, 3-AMP, 5-AP and 5-AMP inhibited the incorporation of [3H]thymidine into DNA without affecting the rate of incorporation of [3H]uridine into RNA. The uptake and incorporation of [14C]cytidine into cellular ribonucleotides and RNA, respectively, were not decreased by 3-AP or 3-AMP; however, the incorporation of cytidine into DNA via ribonucleotide reductase was inhibited markedly. Thus, a pronounced decrease in the formation of [14C]deoxyribonucleotides from radioactive cytidine occurred in the acid-soluble fraction of 3-AP- and 3-AMP-treated L1210 cells. Consistent with an inhibition of DNA replication that occurred at relatively low concentrations of 3-AP and 3-AMP, cells gradually accumulated in the S-phase of the cell cycle; at higher concentrations of 3-AP and 3-AMP, a more rapid accumulation of cells in the G0/G1 phase of the cell cycle occurred, with the loss of the S-phase population, implying that a second less sensitive metabolic lesion was created by the HCTs. N-Acetylation of 3-AMP resulted in a compound that was 10-fold less active as an inhibitor of ribonucleotide reductase activity and 8-fold less active as an inhibitor of L1210 cell growth. N-Acetylation of either 5-AP or 5-AMP did not alter the inhibitory properties of these compounds. The results obtained provide an experimental rationale for the further development of the HCTs, particularly 3-AP and 3-AMP, as potential drugs for clinical use in the treatment of cancer.

Structural aspects of N-hydroxy-N'-aminoguanidine derivatives as inhibitors of L1210 cell growth and ribonucleotide reductase activity

Previous studies have shown that N-hydroxy-N'-aminoguanidine (HAG) derivatives [RCH = NNHC(= NH)NHOH-tosylate] inhibit ribonucleotide reductase activity and block the growth of leukemia L1210 cells and human colon carcinoma, HT-29, cells in culture. In the current studies, the role of the side chains and the location of the bond of the side chain moiety to HAG were investigated using a new series of HAG derivatives which contained as the R-group--cyclohexyl, phenyl-, pyridyl- or napthyl moieties. The effects of these compounds as inhibitors of L1210 cell growth and ribonucleotide reductase activity were compared with the parent compound. N-hydroxy-N'-aminoguanidine was less inhibitory to ribonucleotide reductase activity and L1210 cell growth than hydroxyurea. The phenyl-HAG compounds which included 1-benzyloxybenzylidene- and 4-cyclohexylmethoxybenzylidene-HAG inhibited CDP reductase with IC50s which ranged from 50-110 microM. 1-Naphthylmethylene-HAG was more inhibitory than 2-naphthylmethylene-HAG and more inhibitory than the phenyl-HAG compounds. 2-Pyridylmethylene-HAG was more inhibitory than 3-pyridylmethylene- or 4-pyridylmethylene-HAG. While HAG inhibited CDP and ADP reductase activities essentially to the same extent, the HAG-derivatives inhibited ADP reductase activity to a greater extent than CDP reductase activity. Cyclohexylmethylene-HAG did not inhibit either L1210 cell growth or ribonucleotide reductase activity. There was good correlation between the inhibition of ribonucleotide reductase activity and L1210 cell growth by these HAG-derivatives. These data indicate that not only is the nature of the side chain substitution important, but also the location of the HAG-moiety on the ring position.

Aldehyde dehydrogenases and their role in carcinogenesis

Aldehydes are highly reactive molecules that may have a variety of effects on biological systems. They can be generated from a virtually limitless number of endogenous and exogenous sources. Although some aldehyde-mediated effects such as vision are beneficial, many effects are deleterious, including cytotoxicity, mutagenicity, and carcinogenicity. A variety of enzymes have evolved to metabolize aldehydes to less reactive forms. Among the most effective pathways for aldehyde metabolism is their oxidation to carboxylic acids by aldehyde dehydrogenases (ALDHs). ALDHs are a family of NADP-dependent enzymes with common structural and functional features that catalyze the oxidation of a broad spectrum of aliphatic and aromatic aldehydes. Based on primary sequence analysis, three major classes of mammalian ALDHs--1, 2, and 3--have been identified. Classes 1 and 3 contain both constitutively expressed and inducible cytosolic forms. Class 2 consists of constitutive mitochondrial enzymes. Each class appears to oxidize a variety of substrates that may be derived either from endogenous sources such as amino acid, biogenic amine, or lipid metabolism or from exogenous sources, including aldehydes derived from xenobiotic metabolism. Changes in ALDH activity have been observed during experimental liver and urinary bladder carcinogenesis and in a number of human tumors, including some liver, colon, and mammary cancers. Changes in ALDH define at least one population of preneoplastic cells having a high probability of progressing to overt neoplasms. The most common change is the appearance of class 3 ALDH dehydrogenase activity in tumors arising in tissues that normally do not express this form. The changes in enzyme activity occur early in tumorigenesis and are the result of permanent changes in ALDH gene expression. This review discusses several aspects of ALDH expression during carcinogenesis. A brief introduction examines the variety of sources of aldehydes. This is followed by a discussion of the mammalian ALDHs. Because the ALDHs are a relatively understudied family of enzymes, this section presents what is currently known about the general structural and functional properties of the enzymes and the interrelationships of the various forms. The remainder of the review discusses various aspects of the ALDHs in relation to tumorigenesis. The expression of ALDH during experimental carcinogenesis and what is known about the molecular mechanisms underlying those changes are discussed. This is followed by an extended discussion of the potential roles for ALDH in tumorigenesis. The role of ALDH in the metabolism of cyclophosphamidelike chemotherapeutic agents is described. This work suggests that modulation of ALDH activity may an important determinant of the effectiveness of certain chemotherapeutic agents.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition of iron uptake in HL60 cells by 2-formylpyridine monothiosemicarbazonato Cu(II)

The electron paramagnetic resonance (EPR) signal of the tyrosyl radical attributed to ribonucleoside diphosphate reductase decreases after treatment of promyelocytic leukemic HL60 cells with 2-formylpyridine thiosemicarbazonato copper(II) (CuL). According to EPR studies, CuL forms adducts with both histidine and cysteine-like Lewis bases associated with isolated membranes from HL60 cells. After the addition of CuL, the EPR signal for the cysteine-like adduct decreases substantially over a 15-min period. The reduction of signal is consistent with oxidation of thiols as shown by an analysis of sulfhydryl content. It is hypothesized that receptor-mediated transferrin internalization is inhibited by oxidation of critical thiols. Since the uptake of 59Fe-transferrin is greatly inhibited by the treatment of HL60 cells with CuL, the reduced uptake of iron by cells, in the presence of CuL, may lead to decreased iron availability for the activity of the M2 subunit of ribonucleotide reductase and a subsequent decrease in the tyrosyl radical signal of the enzyme. Moreover, the intact subunit M2 is no longer detected by EPR, even after the addition of excess iron.

Quantitative structure-activity relationship (QSAR) analysis of the cytotoxicities of aminohydroxyguanidine derivatives and their antiviral activities in vitro

Substituted Schiff bases of 1-amino-3-hydroxyguanidine (SB-HAG) were tested for the first time against noninfected T4 lymphocytes (CEM-6 cells) and the same cell line infected by HIV-1 in vitro. Twenty-one of 23 compounds at micromolar levels did not inhibit the growth of the noninfected T4 cells, suggesting minimal cytotoxicity. The antiviral effects of these compounds in a micromolar concentration range have been shown to be nonsignificant (less than 30%) against HIV-1. Three-dimensional parameter focusing of the physicochemical properties (i.e., log P and VW) and the marginal antiviral activities shows that the marginally active compounds lie in a region different from the inactive compounds. QSAR analysis of the two subsets shows that the cytotoxicity correlates well with the electronic and lipophilic parameters. The results of the QSAR analysis can serve as guidelines for further structural modification of this series of compounds to minimize the cytotoxicity against host cells.

Studies on the mechanisms of inhibition of L1210 cell growth by 3,4-dihydroxybenzohydroxamic acid and 3,4-dihydroxybenzamidoxime

Didox and Amidox inhibit L1210 cell growth in culture. At least one of the mechanisms in the mode(s) of action of the compounds is directed at the ribonucleotide reductase site. Partially purified preparations of ribonucleotide reductase activity are inhibited by Amidox and Didox. The formation of deoxycytidine nucleotides from [14C]cytidine in intact L1210 cells is also blocked. Didox and Amidox cause the decrease in the intracellular pools of the four dNTPs. Hydroxyurea-resistant L1210 cells are not cross-resistant to either Didox or Amidox. These data suggest that Didox and Amidox are not inhibiting ribonucleotide reductase through a mechanism similar to hydroxyurea.

Effect of ribonucleotide reductase inhibitors on the growth of human colon carcinoma HT-29 cells in culture

The effects of ribonucleotide reductase inhibitors on the growth of the human colon carcinoma cell line HT-29 were examined. Inhibitors were chosen for these studies that were specifically directed at each of the subunits of ribonucleotide reductase. The concentrations of drugs required to inhibit the growth of HT-29 cells by 50% (IC50) for hydroxyurea, 2,3-dihydro-lH-pyrazole-[2,3a]imidazole (IMPY), and 4-methyl-5-amino-l-formyl-isoquinoline thiosemicarbazone (MAIQ) were 206, 996, and 3.2 microM, respectively. Although the IC50 for deoxyadenosine alone was greater than 2,000 microM, in the presence of 5 microM erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA), which protects deoxyadenosine from deamination by adenosine deaminase, it was reduced to 112 microM. The IC50 for deoxyguanosine was 1,060 microM. The addition of 8-aminoguanosine to protect deoxyguanosine from phosphorolysis by purine nucleoside phosphorylase did not increase the toxicity of deoxyguanosine in HT-29 cells. The combination of MAIQ or IMPY and deoxyadenosine/EHNA gave strong synergistic inhibition of HT-29 cell growth. The results of these studies indicate that ribonucleotide reductase inhibitors effectively block the growth of human colon carcinoma HT-29 cells and that combinations of inhibitors directed at the individual subunits of reductase result in synergistic inhibition of HT-29 cell growth in culture.

Quenching of tyrosine radicals of M2 subunit from ribonucleotide reductase in tumor cells by different antitumor agents: an EPR study

The inhibition of ribonucleotide reductase (RR) of intact Ehrlich ascites tumor cells by different antitumor agents was compared using EPR spectroscopy. The inactivation of M2 subunit was measured via quenching of the functionally essential tyrosine radical. Inhibitors of different classes, for example, hydroxyurea, pyrogallol, and thiosemicarbazones, differ in their efficiency by three orders of magnitude. Most effective inhibition was found for isoquinoline-1-aldehyde-thiosemicarbazone (IQ-1) with an IC50 value of 0.18 microM. Inhibition of RR inside tumor cells is comparable with that reported for isolated enzymes.

Synthesis, spectroscopic, and antitumor activity of metal chelates of S-methyl-N-(l-isoquinolyl)-methylendithiocarbazate

Complexes of Mn(III), Fe(III), Fe(II), Co(III), Ni(II), Cu(II), Zn(II), and Pt(II) with S-methyl-N-(l-isoquinolyl) methylendithiocarbazate (N-N-SH) were isolated and characterized by elemental analysis, conductance measurement, magnetic susceptibilities, and spectroscopic studies. On the basis of these studies, a highly distorted, high-spin, chloro-bridged, polymeric octahedral structure for [Mn(N-N-S)Cl2]; a distorted, low-spin, monomeric octahedral structure for [Fe(N-N-S)2]; a distorted, high-spin, octahedral structure for [Ni(N-N-S)2]; and a square-planar structure for [M(N-N-S)X] (M = Ni, Cu, Pt or Zn and X = Cl- or -OAc) are suggested. With Fe(III), the complex [Fe(N-N-S)2][FeCl4] was isolated while the Co(II) was oxidized to yield the Co(III) ion as [Co(N-N-S)2]2[CoCl4]. All these complexes were screened for their antitumor activity against P 388 lymphocytic leukemia test system in mice. Except for Mn(III), Fe(III), and Co(III) complexes, all were found to possess significant activity; the Cu(II) and Zn(II) complexes showed a T/C% value of 160 and 195, respectively, at their optimum dosages.

In vitro antiproliferative activity of 2'-(2-hydroxyphenyl)-2'-thiazoline-4'-carboxylic acid and its methyl ester on L1210 and P388 murine neoplasms

The activity of three iron chelators, methyl [2'-(2-hydroxyphenyl)-2'-thiazoline-4'-carboxylate] (MTL); 2'-(2-hydroxyphenyl)-2'-thiazoline-4'-carboxylic acid (TFAL); and 2-hydroxyphenyl-imido-ethyl-ether (Imidate), regarding antiproliferative, cytocidal, and cell-cycle effects are reported and compared with hydroxyurea (HU). In vitro, against L1210 and P388 murine neoplasms, MTL and TFAL displayed substantially greater antiproliferative activity than HU, although Imidate displayed no appreciable activity. MTL also induced a statistically more complete G1/S cell-boundary block than did HU at equimolar concentrations (100 microM). The IC50 values produced by MTL and TFAL were low enough (less than or equal to 20 microM) to warrant further testing of these chelators as potential antineoplastic agents.

Properties of N-hydroxy-N'-aminoguanidine derivatives as inhibitors of mammalian ribonucleotide reductase

In previous studies, N-hydroxy-N'-aminoguanidine (HAG) derivatives were demonstrated to suppress growth and clonogenicity of tumor cells which correlated with the inhibition of ribonucleotide reductase and DNA synthesis. The present work has focused on the properties of five HAG derivatives as inhibitors of the ribonucleotide reductase from Ehrlich ascites tumor cells. HAG derivatives acted as non-competitive inhibitors of ribonucleotide reductase with respect to the substrates CDP and ADP. The apparent Ki values for the various HAG derivatives as inhibitors of CDP reductase ranged from 3.4 to 543 microM. However, the apparent Ki values for these inhibitors with respect to ADP reductase were 2- to 10-fold lower than the respective values for CDP reductase. After a preincubation of HAG derivatives and ribonucleotide reductase in the absence of substrates, an increased inhibition was observed. The activity of the inhibited enzyme could be restored by passage over a Sephadex G-25 column and subsequent incubation with dithioerythritol. The addition of either the non-heme iron subunit or the effector-binding subunit to the intact enzyme in the assay mixture resulted in a diminished inhibition of ADP reduction. Inhibition by HAG derivatives of ribonucleotide reductase activity in the test tube was not enhanced by iron chelators. However, a combination of HAG compounds and iron chelators synergistically inhibited the growth of L1210 cells.

Synthesis and antimicrobial activities of N4-(2-acetoxyethoxymethyl)thiosemicarbazones and N3-(2-acetoxyethoxymethyl)thioureas

A series of thiosemicarbazones and thioureas having an open-chain analogue of the ribosyl group, the 2-acetoxyethoxymethyl moiety, has been synthesized. Significant growth inhibitory activity versus gram-positive and gram-negative organisms, a yeast, and a mold has been found with the 2-acetoxyethoxymethyl derivatives of N-alkyl-, aryl-, and heteroaryl-thiosemicarbazones and thioureas. The molecules may function as inhibitors of ribonucleotide reductase or in utilization of the carbamyl group in pyrimidine biosynthesis.

Inhibition of ribonucleotide reductase and L1210 cell growth by N-hydroxy-N'-aminoguanidine derivatives

A series of N-hydroxy-N'-aminoguanidine derivatives was studied for their effects on L1210 cell growth and ribonucleotide reductase activity. With the twelve compounds studied, there was a good correlation between the inhibition of L1210 cell growth and the inhibition of ribonucleotide reductase activity. The most potent compound required concentrations of only 1.4 and 2 microM for 50% inhibition of L1210 cell growth and ribonucleotide reductase activity respectively. These guanidine analogs specifically inhibited the conversion of [14C]cytidine and deoxycytidine nucleotides in the nucleotide pool and the incorporation of [14C]cytidine into DNA without altering the incorporation of [14C]cytidine into RNA. Ribonucleotide reductase activity in drug-treated cells was reduced markedly. Iron-chelating agents did not either increase or decrease the inhibition caused by the N-hydroxy-N'-aminoguanidine derivatives. No evidence was obtained that these derivatives selectively inactivated one of the subunits of ribonucleotide reductase. These compounds appear to inhibit ribonucleotide reductase by a mechanism different from hydroxyurea or the thiosemicarbazone derivatives.

Combination chemotherapy directed at the components of nucleoside diphosphate reductase

It would be expected that drugs directed at the rate-limiting step in a key metabolic pathway in tumor cell proliferation would provide a useful basis for therapy of neoplasms. Ribonucleotide reductase catalyzes the rate-limiting step in the de novo synthesis of dNTP's for DNA synthesis. Further, ribonucleotide reductase is composed of two non-identical protein subunits (non-heme iron and effector-binding subunits) which can be specifically and independently inhibited. As a result, combinations of drugs specifically directed at each of the subunits of ribonucleotide reductase have been shown to cause synergistic inhibition of L1210 cell growth in culture and synergistic cell kill. This approach offers a novel basis for the design of combination chemotherapy.

Drug action on ribonucleotide reductase

Ribonucleotide reductase catalyzes the rate-limiting step in DNA synthesis. It represents a key metabolic site at which specific inhibitors have been directed as potential antitumor agents. Several different classes of ribonucleotide reductase inhibitors have been generated and studied. Because of the nature of the DNA polymerase reaction in which all four dNTPs are required, the initial velocity vs dNTP concentration curve gives sigmoidal rather than hyperbolic kinetics. As a result, a 50 per cent decrease in ribonucleotide reductase activity causes a decrease in DNA polymerase activity of 75 per cent or greater depending on the ratio of [dNTP] to its Km. This has been demonstrated with theoretical calculations, actual DNA polymerase determinations and precursor studies in intact tumor cells. The structural requirements for a compound to serve as a specific inhibitor of ribonucleotide reductase, either as the non-heme iron or effector-binding subunit, are stringent. Each protein subunit comprising the active enzyme can be specifically and independently inhibited. When combinations of agents, each directed at one of the subunits of ribonucleotide reductase, are used, strong synergistic inhibition of L1210 cell growth and synergistic cytotoxicity result.

Effects of 2-formylpyridine monothiosemicarbazonato copper II on red cell components

1-Formylpyridine monothiosemicarbazonato copper II (CuL+) is readily taken up by red cells and is initially bound to glutathione and hemoglobin. Glutathione was depleted within 5 hr of incubation, presumably by oxidation mediated by CuL+ and O2 with concomitant generation of toxic oxygen species. Cupric ion was slowly transferred from CuL+ to hemoglobin within about 7 hr and hemoglobin was oxidized until the major form prevailing after 10 hr was alpha 2 beta 2+. Little increase in hemolysis due to addition of CuL+ dissolved in the radical scavenger dimethyl sulfoxide was observed with prolonged incubation. Strong inhibition of red cell hexokinase by CuL+ was observed when the enzymes in red cell lysates and hemoglobin-free red cell lysates were examined. CuL+ was also an effective inhibitor of yeast hexokinase. However, the inhibitory effect of CuL+ within the red cells was less pronounced. It is suggested that even though intracellular accumulation of CuL+ creates an oxidizing environment and is potentially capable of inhibiting thiol enzymes such as hexokinase, protective effects are exerted in the red cell by the presence of hemoglobin, of radical scavengers, and of high levels of enzymes that detoxify toxic oxygen species.